Internal combustion engines may operate in a variety of combustion modes. One example mode is homogeneous charge compression ignition (HCCI), wherein an air and fuel mixture achieves a temperature where combustion occurs by autoignition without requiring a spark being performed by a sparking device. In some conditions, HCCI may have greater fuel efficiency and reduced NOx production compared to other combustion modes. However, combustion modes such as HCCI may be limited under some conditions, since a substantial amount of heat and pressure may be needed to produce combustion. Further, transitions into and out of the HCCI mode may present various control issues.
One approach to enable operation in a variety of combustion modes is described in U.S. 2005/0183693. In this example, a cylinder with cam profile switching is used to provide operation in both HCCI and SI combustion.
However, the inventors herein have recognized a disadvantage with such an approach. For example, during the transient conditions of SI/HCCI transitions in the above system, the cylinders may experience incorrect cylinder air amounts, residual amounts, and/or fuel charge amounts.
In one approach, the above issues may be addressed by a method of operating an engine having at least one cylinder, comprising: directing a first air stream to a first intake valve of the cylinder; directing a second, separate, air stream to a second intake valve of the cylinder, said first stream at a higher temperature than said second air stream; during a first mode, operating with said first intake valve providing more airflow than the second intake valve; varying a timing of at least one of the first and the second intake valves during the first mode when transitioning to a second mode; and during the second mode, operating with said first intake valve providing less airflow than the second intake valve, and varying a timing of at least one of the first and second intake valves after transitioning from the first mode.
In this way, in the HCCI mode of operation, the mass and temperature of air in the cylinder and the intake cam timing (e.g. the intake valve closing—IVC—angle) are varied to achieve a high or the highest possible fuel economy and/or emissions at the given engine speed and torque (e.g. as determined by the amount of fuel burned). Likewise, in the SI mode the spark timing and intake cam timing may be varied for the same purpose. To achieve smooth transients from one mode to another and reset the after treatment system if necessary, a set of operations may be performed that includes positioning the throttles and cam timing prior to the mode switch and restoring the optimal schedules appropriate for the new mode after the switch. This approach takes advantage of the relatively low sensitivity to the IVC of the amount of air trapped in the cylinder when IVC is close to TDC relative to the sensitivity when IVC is close to the middle of the compression stroke. Thus, this approach is enabled by the longer duration of the dominant (cold stream) intake valve in the SI mode compared to the valve duration(s) in the HCCI mode. By varying the valve timing of at least one of the intake valves, the absolute amount of air and/or the initial charge temperature may be rapidly varied, thereby providing improved operation when changing combustion modes. Further, by providing at least some heated air during a spark ignition mode, a ready reserve of heated air may be maintained.
Note that in the above approach, cam actuated valves may be used, where various types of cam lobes may be connectable to an intake valve. For example, a tappet may be connectable to one of a plurality of cam lobes, wherein a particular cam lobe is selected by varying the position of the cam lobe via cam profile switching. In another approach, a switchable tappet may be reconfigured to provide different contact points for selecting one of a plurality of cam lobes.
In another embodiment, the method may further include varying an amount of fuel delivered to the cylinder during the transition in response to the varied timing. In this way, it is possible to account for the variation in fuel pushback during the timing variation of the transition.
Note that various modes may be provided such as, for example, homogeneous charge compression ignition, compression ignition, homogeneous spark ignition, stratified spark ignition, and spark assisted compression ignition, among others. Further, an air stream of a higher temperature may be provided through heat addition via a heat exchanger and/or by cooling the other intake air stream to achieve a lower temperature.